This invention relates to the technical field of density correction that will be a basis of performing an appropriate color reproduction in accordance with an original image and color correction which is capable of obtaining the appropriate color reproduction by correcting unevenness of spectral sensitivity characteristic and other various characteristics of an image reading apparatus (scanner) when the original image is photoelectrically read.
At present, most of the images recorded on photographic films such as negatives and reversals (which are hereinafter referred to as xe2x80x9cfilmsxe2x80x9d) are printed onto light-sensitive materials (photographic papers) by a technique generally called xe2x80x9cdirect exposurexe2x80x9d (analog exposure) in which the light-sensitive materials are exposed with the light projected from the films.
Reflection original such as printed matters, photographs or the like have also commonly been reproduced by means of analog exposure.
For the former case, printing apparatus which adopt digital exposure have recently been commercialized. In this xe2x80x9ccolor digital printerxe2x80x9d, the image recorded on a film is read photoelectrically and converted into digital signals, which are subjected to various kinds of image processing to produce recording image data; a light-sensitive material is scanned and exposed with recording light modulated in accordance with the image data, thereby recording a (latent) image which is then made to a (finished) print (photograph).
Also for the latter case, an apparatus employing a digital system has been commercialized as in a same manner as in the former case. Moreover, in the above-mentioned color digital printer, a so-called composite printer which can produce a copy (print) from the reflection original such as a printed matter, a photography or the like has recently been commercialized.
This composite printer is basically composed of a transparent original reading scanner for photoelectrically reading an image recorded on a film with an image sensor, a reflection original reading scanner for photoelectrically reading an image recorded on the reflection original such as a printed matter, a photograph or the like with an image sensor, an image processing apparatus for subjecting the image data read with those scanners or image data supplied from a digital camera or the like to preset image processing and making the image data to image data for recording the image, that is, setting an exposure condition based on the image data, a printer (image recording apparatus) for recording a latent image by scanning and exposing a light-sensitive material with, for example, a light beam scanning in accordance with the image data outputted from the image processing apparatus and a processor (developing apparatus) for subjecting the light-sensitive material having been exposed with the printer to development processing and for outputting a print on which the image is reproduced.
In such a composite printer, since the image can be read as digital image data and the exposure condition used in printing can be determined by subjecting the digital image data to image processing, a print of high quality which can not be obtained by the conventional direct exposure can be obtained, for example, from the image on the negative film, by properly executing the correction of a washed-out highlight and a dull shadow due to photography with back light or an electronic flash, sharpening processing and the like. Moreover, a plurality of images can be composited to a single image or one image can be split into segments through the image data processing. As a result, prints can be outputted after images are freely subjected to editing and processing in accordance with applications.
Further, also as to the image of the reflection original, various image processing can be performed to produce a high-quality print.
In the transparent original reading scanner or the reflection original reading scanner of the above composite printer, reading light is incident on an original, and projected light which has passed through the original (film) or reflected light which has been reflected from the original printed matter), is read with an image sensor such as a CCD sensor or the like thereby photoelectrically reading an original image.
In the case of a color original, the projected light or the reflected light of the original is processed with red (R), green (G) and blue (B) color filters or the reading light processed with the R, G, and B color filters is incident on the original, whereby the original image is read by being separated into the three primary colors R, G and B.
In the above situation, when scanners of the same type read the same original, they must obtain the same image signals (image data).
However, instrumental errors exist in light sources, color filters, image sensors and the like which are mounted on scanners. As a result, these scanners have different spectral sensitivity characteristics each other even if they are of the same type. Accordingly, since resultant image signals are influenced by the spectral sensitivity characteristic of each scanner, images having different color tints are produced with each scanner even if visible images are reproduced using the image signals. Thus, appropriate colors can not be always reproduced in accordance with the original, that is, an image of high-quality can not be always reproduced.
Similar problem exists with density. Even in a monochrome image, since the instrumental errors exist in light sources and image sensors installed in the scanner from one another, the image signals to be obtained differ from one scanner to another scanner so that an appropriate density can not be always reproduced in accordance with the original.
A first object of the invention is to solve the above problems of the prior art and to provide a method for correcting a density characteristic which will be a basis of performing an appropriate color reproduction in accordance with an original image by correcting unevenness of various characteristics of a scanner caused by a light source, an optical filter, an image sensor or the like and reading errors caused by this unevenness.
A second object of the invention is to solve the above problems of the prior art and to provide a method for correcting a color which is capable of obtaining a constant color reproduction by suitably correcting the unevenness of spectral sensitivity of the scanner caused by the light source, the color filter, the image sensor or the like and the reading errors caused by this unevenness.
In order to attain the above-described first object, a density characteristic correcting method according to a first aspect of the invention comprises:a first step of measuring a reference original with a preset measuring instrument to obtain a measured result and storing the thus obtained measured result in a storing means; a second step of reading the reference original with an image reading apparatus (scanner) which is a target to be corrected and obtaining a statistic by analyzing image signal values in a preset position within an image region of the reference original; a third step of reading out the measured result stored in the storing means; and a fourth step of calculating an input density characteristic correction parameter from the static obtained by the second step and the measured result read by the third step.
It is preferable that the density characteristic correcting method according to the invention creates a shading correction parameter before reading in the second step is performed and uses the thus created shading correction parameter when the reading in the second step is performed.
It is also preferable that in the density characteristic correcting method according to the invention, the image signal values obtained by measuring the reference original with the preset measuring instrument and a position information thereof are stored in the storing means as the measured result.
Moreover, it is further preferable that the input density characteristic correction parameter in the density characteristic correcting method according to the invention is a lookup table (LUT) which is determined such that an error between the measured result obtained in the first step and the statistic obtained in the second step is minimized.
The image signal value preferably has a higher resolution of bits than an image signal value for generally forming an image has and it is preferable that the measured result includes a measured value and a position thereof and that the measured value is any one of status M density (printing density), status A density (colorimetric density), R, G and B values measured on the basis of X, Y and Z colorimetric values and the image signal value by an reference input device (scanner). The statistic is preferably a mean value or a median value within a preset region. Calculation of the lookup table (LUT) is preferably to calculate a polynomial approximate expression so as to use a polynomial coefficient thereof as a parameter. It is preferable that, when an error minimization operation is executed, weighting is performed putting importance on a lower density. It is preferable that the reference original spectrally has a nearly flat characteristic and that it is an ND filter for the transparent original scanner and a color chart such as a gray patch or a gray chart for the reflection original scanner. Moreover, the reference original preferably includes a low density portion (density of 0.2 or less). The reference original preferably includes a high density portion (density of 2.5 or more for transparent original; and density of 1.5 or more for reflection original).
In order to attain the above-described second object, a color correcting method according to a second aspect of the invention comprises:a first step of measuring a reference original with a preset measuring instrument and then storing the thus obtained measured result in a storing means; a second step of reading the reference original with an image reading apparatus (scanner) which is a target to be corrected and obtaining a statistic by analyzing image signal values in a preset position within an image region of the reference original; a third step of reading out the measured result stored in the storing means; and a fourth step of calculating an input color correction parameter from the statistic obtained in the second step and the measured result read in the third step.
It is preferable that the color correcting method according to the invention creates a shading correction parameter before reading in the second step is performed and uses the thus created shading correction parameter when the reading in the second step is performed, and that the color correction method creates an input density correction parameter for an original which spectrally has a nearly constant reflectance transmittance before reading with the image reading apparatus in the second step is performed thereby using the thus created input density correction parameter when the reading in the second step is performed.
Moreover, it is also preferable that in the color correcting method according to the invention, the image signal values obtained by measuring the above-mentioned reference original with a preset measuring instrument and position information thereof are stored in the above-mentioned storing means as the measured result.
Furthermore, it is further preferable that in the color correcting method according to the invention, calculation of the input color correction parameter in the fourth step comprises the steps of:selecting a parameter which has a smallest error from among previously obtained candidate parameters; calculating a correction parameter which minimizes an error amount to be generated from the thus selected parameter; and combining the selected parameter and the thus calculated correction parameter.
It is still further preferable that as the reference original, a same kind of reference original as that of the original which is the target to be read is used.
One-dimensional LUT (lookup table) is preferably calculated by targeting only an achromatic color within the reference original so as to absorb an error between a measured value of the reference original and a correction value obtained by the above-calculated parameter. It is preferable that in the calculation of one-dimensional LUT, a polynomial approximate expression is calculated and an approximation coefficient thereof is used as a parameter. It is preferable that the candidate parameter is a quadratic matrix coefficient; the calculation correction parameter is a linear matrix coefficient; and the color correction parameter is the quadratic matrix. It is preferable that the quadratic matrix of the color correction parameter is expanded into a three-dimensional LUT (lookup table) (by CPU or the like) which is then used as the color correction parameter. This three-dimensional LUT and the one-dimensional LUT are preferably combined to be another three-dimensional LUT which is then used as the color correction parameter.
The image signal value preferably has a higher bit resolution than an image signal value for generally forming an image has. It is preferable that the measured result includes the measured value and a position thereof and that the measured value is any one of status M density (printing density), status A density (colorimetric density), R, G and B values measured on the basis of the X, Y and Z colorimetric values and the image signal value by an reference input device (scanner). The statistic is preferably a mean value or a median value within a preset region. It is preferable that an input color correction (separation) parameter is calculated using the reference original coincided with (the type of) the target to be read. In other words, it is preferable that input color correction (separation) parameters are calculated using associated reference originals:the reference original of a negative film when the negative film is read; the reference original of a reversal film when the reversal film is read; the reference original of a photograph when the photograph original is read; and the reference original of a printed matter when the printed matter is read.
In each of above aspects, a plurality of reference originals may be used as the above-mentioned reference original.
In this case, it is preferable that a number of the above plurality of reference originals and an order thereof to be used are previously determined, the measured results corresponding to the above-mentioned plurality of reference originals are stored in the above-mentioned storing means in the thus determined number and order to be used, the stored measured result corresponding to the reference original is read out from the above-mentioned storing means in the above-mentioned previously determined order to be used, as well as the above-mentioned statistic is obtained by reading the above-mentioned corresponding reference original in accordance with the above-mentioned previously determined order to be used by means of the above-mentioned image reading apparatus, the obtained statistic and the above-mentioned read-out corresponding measured result are sequentially accumulated in the previously determined order, and, after the above-mentioned statistics and the above-mentioned measured results corresponding to the above-mentioned previously determined number of reference originals are obtained, at least one of the above-mentioned input density characteristic correction parameter and the above-mentioned input color correction parameter is calculated. It is preferable herein that, before the above-mentioned reference originals is used, the above-mentioned plurality of reference originals and the order thereof to be used are displayed and notified outside.
Moreover, it is preferable that a number of the above-mentioned plurality of reference originals and an order thereof to be used as well as measured results corresponding to these plurality of reference originals are stored previously in the above-mentioned storing means, the number and the order to be used of the above-mentioned plurality of reference originals are read out by a control means; the above plurality of reference originals to be used and the order thereof to be used are displayed to be notified outside, the above-mentioned measured result corresponding to the reference original is read from the above-mentioned storing means in the above-mentioned order to be used, as well as the above-mentioned statistic is obtained by reading the above-mentioned corresponding reference originals in accordance with the above-mentioned order to be used by means of the above-mentioned image reading apparatus; the obtained statistic and the above-mentioned read out corresponding measured result are sequentially accumulated in the order to be used, and, after the above-mentioned statistics and measured results corresponding to the above-mentioned read-out number of reference originals are obtained, at least one of the above-mentioned input density characteristic correction parameter and the above-mentioned input color correction parameter is calculated.
Further, it is preferable that the measured results corresponding to the plurality of reference originals in which the measured results are correlated with the reference originals are stored in the storing means, an information of the of the reference original to be used is obtained, the measured result corresponding to the reference original is read out from the storing means, as well as the statistic is obtained by reading the reference original to be used with the image reading apparatus, the thus obtained statistic and the read-out corresponding measured result are sequentially accumulated, and at least one of the input density characteristic correction parameter and the input color correction parameter from the accumulated statistics and the accumulated measured results.
In this case, it is preferable that the information of the reference original to be used is inputted from outside with a information inputting means and that at least one of the input density characteristic correction parameter and the input color correction parameter is calculated based on a calculating instruction of the correction parameter which is inputted with the information input means from outside
Furthermore, it is preferable that the above-mentioned reference original is provided with a bar code, this bar code is read by a bar code reader, and the above-mentioned measured result is read from the above-mentioned storing means in accordance with the read bar code.
It is also preferable that an information representing the above-mentioned measured result corresponding to the reference original and, optionally, a presence or absence of calculation of at least one of the above-mentioned input density characteristic correction parameter and the above-mentioned input color correction parameter are recorded on a portion of the above-mentioned reference original using at least one of a numeral, a patch density, a pattern and a bar code, the above-mentioned measured result information and, optionally, the presence or absence of the calculation of the above-mentioned correction parameter are read as an image information at the same time when the above-mentioned reference original is read with the above-mentioned image reading apparatus, the above-mentioned read image information is analyzed and judged, and the above-mentioned corresponding measured result is read, optionally the above-mentioned presence or absence of the calculation is read and the above-mentioned corresponding measured result is accumulated in case of the above-mentioned absence of the calculation while at least one of the above-mentioned input density characteristic correction parameter and the above-mentioned input color correction parameter is calculated in case of the above-mentioned presence of the calculation.